1. Field of the Invention
The present invention relates to a data transmission apparatus for transmitting data from a plurality of data generating blocks to a common bus, which is applicable, for example, to a picture signal processing apparatus having, as its data generating blocks, video-signal data generating means and character data generating means.
2. Description of the Prior Art
So far, in a data transmission apparatus structured such that a plurality of data generating blocks therein are adapted to be able to independently transmit data to a data bus, there has been provided, in general, a bus control block for deciding which data generating block is granted permission to use the bus when two or more of the data generating blocks have simultaneously requested permission to transmit their data.
A prior art bus control system as described above is disclosed in the Japanese Patent Application Publication No. 46099/82. The disclosed bus control system is, as shown in FIG. 3, a multiprocessor system including a plurality of microprocessors processing data through accessing a bused common memory area, in which system the plurality of microprocessors are divided into m groups of minor multiprocessor systems, these divided groups are connected with each other through a bus controller, each divided group is provided with at least one bused common memory area, and the plurality of microprocessors belonging to the group are enabled to access the bused common memory area independently of each other. Further describing the system with reference to FIG. 3, reference numerals 31A to 31D denote the minor multiprocessor systems, 32 denotes a bus controller, and 33 denotes a control processor for the entire system. In the described bus system, when all the minor multiprocessor systems 31A, 31C, and 31D wish to access the minor multiprocessor system 31B, the bus controller 32 is adapted, for example, to decide the order of priority and permit a specific minor multiprocessor system to access the minor multiprocessor system 31B.
In the prior art data transmission apparatus as described above, the switching to a specific data generating block for allowing the same to output data to the bus was only possible when there was provided a pattern generated within the bus control block or a pattern that could be identified by the bus control block. Therefore, it was impossible for the apparatus to make the above switching in response to the pattern whatever the data output from a specific data generating block would take and without the process of identification of the pattern by the bus control block. In the case, for example, where one of the data generating blocks was a video-signal data generating block and the other was a character-data generating block, there was such a problem that it was impossible for the prior art apparatus to cause the video-signal data and the character data output from both the blocks to be transmitted through a common bus.